Wells are generally drilled into the ground or ocean bed to recover natural deposits of oil and gas, as well as other desirable materials, that are trapped in geological formations in the Earth's crust. A well is typically drilled using a drill bit attached to the lower end of a “drill string.” Drilling fluid, or “mud,” is typically pumped down through the drill string to the drill bit. The drilling fluid lubricates and cools the drill bit, and it carries drill cuttings back to the surface in the annulus between the drill string and the borehole wall.
For successful oil and gas exploration, it is necessary to have information about the subsurface formations that are penetrated by a borehole. For example, one aspect of standard formation evaluation relates to the measurements of the formation pressure and formation permeability. These measurements are essential to predicting the production capacity and production lifetime of a subsurface formation.
One technique for measuring formation properties includes lowering a “wireline” tool into the well to measure formation properties. A wireline tool is a measurement tool that is suspended from a wire as it is lowered into a well so that is can measure formation properties at desired depths. A typical wireline tool may include a probe that may be pressed against the borehole wall to establish fluid communication with the formation. This type of wireline tool is often called a “formation tester.” Using the probe, a formation tester measures the pressure of the formation fluids, generates a pressure pulse, which is used to determine the formation permeability. The formation tester tool also typically withdraws a sample of the formation fluid for later analysis.
In order to use any wireline tool, whether the tool be a resistivity, porosity or formation testing tool, the drill string must be removed from the well so that the tool can be lowered into the well. This is called a “trip” downhole. Further, the wireline tools must be lowered to the zone of interest, generally at or near the bottom of the hole. A combination of removing the drill string and lowering the wireline tools downhole are time-consuming measures and can take up to several hours, depending upon the depth of the borehole. Because of the great expense and rig time required to “trip” the drill pipe and lower the wireline tools down the borehole, wireline tools are generally used only when the information is absolutely needed or when the drill string is tripped for another reason, such as changing the drill bit. Examples of wireline formation testers are described, for example, in U.S. Pat. Nos. 3,934,468; 4,860,581; 4,893,505; 4,936,139; and 5,622,223.
As an improvement to wireline technology, techniques for measuring formation properties using tools and devices that are positioned near the drill bit in a drilling system have been developed. Thus, formation measurements are made during the drilling process and the terminology generally used in the art is “MWD” (measurement-while-drilling) and “LWD” (logging-while-drilling). A variety of downhole MWD and LWD drilling tools are commercially available. Further, formation measurements can be made in tool strings that do not include a drill bit at a lower end thereof, but which are used to circulate mud in the borehole.
MWD typically refers to measuring the drill bit trajectory as well as borehole temperature and pressure, while LWD refers to measuring formation parameters or properties, such as resistivity, porosity, permeability, and sonic velocity, among others. Real-time data, such as the formation pressure, allows the drilling company to make decisions about drilling mud weight and composition, as well as decisions about drilling rate and weight-on-bit, during the drilling process. The distinction between LWD and MWD is not germane to this disclosure.
Formation evaluation while drilling tools capable of performing various downhole formation testing typically include a small probe or pair of packers that can be extended from a drill collar to establish hydraulic coupling between the formation and pressure sensors in the tool so that the formation fluid pressure may be measured. Some existing tools use a pump to actively draw a fluid sample out of the formation so that it may be stored in a sample chamber in the tool for later analysis. Such a pump may be powered by a generator in the drill string that is driven by the mud flow down the drill string.
However, as one can imagine, multiple moving parts involved in any formation testing tool, either of wireline or MWD, can result in equipment failure or less than optimal performance. Further, at significant depths, substantial hydrostatic pressure and high temperatures are experienced thereby further complicating matters. Still further, formation testing tools are operated under a wide variety of conditions and parameters that are related to both the formation and the drilling conditions.